1. Field of the Invention
The present invention relates to an exposure control method for a camera and an apparatus for the method.
2. Background Art
In the case of an ordinary negative film, even if a picture frame is not exactly taken at a proper exposure value, it is possible to correct the image by controlling print-exposure so as to make a reasonable print. However, in an instant camera using a self-developing film, it is impossible to correct exposure afterward. Therefore, it is very important for instant photography to expose the film at a proper value.
In view of this, an exposure control method, called mix-exposure method, is suggested, for example, in U.S. Pat. No. 4,530,585 and JPB 5-14256, wherein an amount of flash light is always automatically projected even to a bright subject. In this method, a program shutter is controlled to obtain 80% of a necessary exposure amount from ambient light, while the flash light is added to make up for the remaining 20% of the proper exposure value, on the assumption that a proper exposure value (Ev) is determined in correspondence with a light value (Lv). According to the known method, even though the exposure value is calculated based on an average brightness, which is more or less affected by a background or subsidiary subjects, a satisfactory exposure of the main subject may be achieved in most cases, because the brightness of the main subject, mostly the nearest subject, is supplemented with the flash light.
Recently, cameras have been known that divide a photographic scene into several areas when measuring subject brightness. An exposure value is calculated based on respective brightness values of these areas after weighting each value with an appropriate coefficient. At that time, it is determined from the divisional brightness values, whether the scene is front-lighted or back-lighted. If the scene is back-lighted, an optimum exposure value can be preferably determined based mainly on brightness values of those areas which correspond to a main subject. In day-light synchronized photography, it is possible to obtain an almost satisfactory exposure with respect to both a main subject and a subsidiary subject by controlling shutter speed and aperture value based on a background brightness and illuminating a main subject with a supplementary flash light.
However, to determine an optimum exposure value in the divided brightness measurement with more accuracy, it is necessary to increase the number of divisional areas and weight the respective brightness values by using a complex calculation in accordance with the brightness distribution pattern. Even such a complex and expensive method is not always able to provide an optimum exposure of the main subject, because of wide variety of actual photographic scenes.
Also in the known mix-exposure method, since the mix rate of flash light to ambient light is determined based on an exposure value derived from an average brightness value of the whole area of the photographic scene, consequent photo-prints may contain under-exposed main subjects especially when the main subject is back-lighted or distant.
To achieve a reasonable exposure for any scene with a simpler and economic method, a camera has been known which adopts a photometric device having a central brightness measuring element and a peripheral brightness measuring element for measuring a central brightness from a central area of the scene and a peripheral brightness from a peripheral area. Because the main subject such as a human subject is very often located in the center area of the scene, the two-area divided brightness measuring method facilitates discrimination between front-lighted and back-lighted scenes, and permits controlling exposure mainly in accordance with the central brightness value.
However, since the central brightness value has a large effect on the exposure control especially for back-lighted scenes in this method, it is important to design the central brightness measuring element so as to pick up light from the main subject at a high probability. If the central brightness measuring element is too small, it tends to miss the main subject. If the central brightness measuring element is too large, it tends to cover beyond the main subject to a larger extent. In either case, an optimum exposure value could not be derived from the central brightness value.